Molecular probe dynamics and free volume heterogeneities in n-propanol confined in a regular MCM-41 matrix by ESR and PALS

Abstract

A combined investigation of the spin probe TEMPO mobility and the free volume holes in n-propanol (n-PrOH) confined in a regular virgin MCM-41 matrix by means of ESR or PALS techniques, respectively, is reported.

Confined Effects on Structural Isomers in the MCM-41-SIL Matrix as Seen by Extrinsic Probes via PALS and ESR: n-Butanol vs tert-Butanol

“…At the lowest temperature of our measurements (100 K), the anisotropic hyperfine splitting (hfs) constant of TEMPO in the bulk n -BuOH is 2 A zz ′ (100 K) = 72 G, while for the bulk t -BuOH, we found a significantly lower value of 69 G. The first value agrees with the previous one for n -butanol, and moreover, it is consistent with those for n -alkanols doped with TEMPO, such as n -propanol, , as well as with very similar nitroxides, i.e., di- tert -butyl nitroxide (DTBN) in n -butanol. , As has been generally found, this value is higher than the typical one for nonpolar organic media due to both the polarity and hydrogen-bonding interactions between the polar TEMPO spin probe and the polar protic organics. , On the other hand, the smaller value for t -BuOH is rather close to the typical value of 68 G for nitroxides in the nonpolar aliphatic hydrocarbons. ,,,, This appears to be quite consistent with the micellelike cluster picture of the liquid t -BuOH with the centering of the hydrophilic hydroxyl HO groups of the molecules, which are involved in intermolecular H-bonds and surrounded by a hydrophobic shell formed by the nonpolar CH 3 groups . Consequently, the polar TEMPO molecules appear to be dominantly surrounded by the nonpolar methyl groups of the t -BuOH molecules mimicking the nonpolar surrounding at low temperatures in the solid state.…”

“…A similar situation is valid also for the o -Ps dispersion. In addition, some further features consisting of the presence of basically four regions A–D with distinct thermal behavior of o -Ps annihilation parameters can be distinguished as usually found for many amorphous organics. ,,− They can be described by the following three characteristic PALS temperatures, depicted as T g PALS ( c ) ≅ 180 K, T b1 L ( c ) ≅ 230 K, and finally T b2 L ( c ) ≅ 265 K. The ascription of the first value to the glass-to-liquid transition is strongly supported by the fact that the relative characteristic PALS temperatures T b1 L ( c )/ T g PALS ( c ) = 1.28 and T b2 L ( c )/ T g PALS ( c ) = 1.47reach the typical values observed for amorphous substances. ,,− It is useful to recall that the T b1 L effect in amorphous organics is related to a crossover of the amorphous phase between its strongly and weakly supercooled liquid states, ,,, while the plateau phenomenon is the specific effect of the PALS technique, which can be connected with the formation of the bubblelike character of the free volume at relatively higher- T values with the corresponding T g values. , On the other hand, the atypically high ratios for n -BuOH, i.e., T b1 L ( c )/ T g PALS ( c ) = 1.87 and T b2 L ( c )/ T g PALS ( c ) = 2.1, are associated with a rather distinct origin of the changes connected with the phase transformations, as supported by the DSC data. Finally, it is of interest to mention that the so-ascribed T g PALS ( c ) ∼ 180 K is in good agreement with the aforementioned two estimations of the glass-to-liquid transition temperatures T g DMS ( b ) = 173 and 180 K for amorphous t -BuOH from special DMS experiments. , On the other hand, the apparent absence of the typical stepwise effect in our DSC response for the confined t -BuOH/MCM-41-SIL system might be related to both the large mean free volume hole sizes and the relative very flat bend effect in the τ 3 vs T dependence together with the very pronounced free volume distribution.…”

“…The τ 3 – T and σ 3 – T plots for the confined n -BuOH/MCM-41-SIL system as obtained under similar measuring conditions with one exception for mode4 are displayed in Figure b. At first sight, the PALS response of n -BuOH confined in the MCM-41-SIL matrix exhibits features rather typical for amorphous organicssee, e.g., refs , , and . In the low- T region A, the o -Ps lifetime, τ 3 ( c ), and its dispersion, σ 3 ( c ), are larger by about 15%, but their thermal expansion reflecting the free volume one is smaller compared to the glassy amorphous n -BuOH in the unlimited bulk state formed by the initial fast cooling.…”

“…On increase in temperature of the spin system n -BuOH/TEMPO, several regions of distinct thermal behavior of the 2 A zz ′ ( T ) parameter defining the corresponding characteristic ESR temperatures can be found. Thus, the first slight decrease was observed above T X1 slow ( b ) ≅ 115 K, followed by the small increase above T X2 slow ( b ) ≅ 140 K and then by the decrease above T X3 slow ( c ) ≅ 160 K. The low- T branch of the 2 A zz ′ ( T ) plot is finished by a stronger reduction above ∼175 K followed by a sharp reduction quantified by the main characteristic ESR temperature describing a transition between the slow- and fast-motion regimes at T 50G ( b ) = 188 K. This quantity is operationally defined by the 2 A zz ′ ( T 50G ) = 50 G relationship, at which the characteristic correlation time of the spin probe reorientation τ c ( T 50G ) reaches a few nanoseconds. − ,, Note that this most pronounced characteristic ESR temperature lies close to the melting temperature T m DSC . Next, in the fast-motion regime, above ∼220 K, the constant value of 34.2 G is achieved.…”

“…Evaluations of the ESR spectra were performed in terms of the anisotropic hyperfine splitting (hfs) constant, 2 A zz ″ (100 K), , and by the outermost line separation of the triplet signal, 2 A zz″ ( T ), as a function of temperature . The isotropic hyperfine splitting constant A N parameter was evaluated at RT in a standard way. , Subsequently, the basic characteristic ESR temperature T 50G and some further ones situated within slow- and/or fast-motion regimes T Xi slow and T Xi fast , were determined. In addition, spectral simulations on all four investigated systems at several selected temperatures were carried out using the standard nonlinear square line (NLSL) routine, and the corresponding dynamic and population parameters of the stable free radical of nitroxide type, i.e., correlation times τ c slow and τ c fast and relative fractions F slow and F fast of the slow and fast components, respectively, were estimated.…”